The present invention relates to the construction of microwave filters having sharp frequency selectivity together with a flat or nearly-flat group delay characteristic. A filter of this type is useful for instance for use as a channel filter for the demultiplexers in the multibeam payload of a baseband-processing SS-TDMA communication satellite in order to limit noise and adjacent channel interference prior to demodulation.
The required specifications for a bandpass filter are usually divided into three groups: those concerning out-of-band rejection and in-band amplitude linearity, those for group delay and those for input/output match (return loss). The ideal filter will have flat group delay from 0 to infinite frequency, a rectangular amplitude response and perfect match (i.e. infinite return loss) at the ports. Such characteristics in a passive finite device are impossible though and the best that can be done is to approach them through devices which increase in complexity the nearer the ideal is approached. Chebychev filters are useful for satisfying tight rejection and match specifications, but always at the expense of severe group delay non linearities particularly at the band edges. Self-equalization partly solves the problem by linearizing the central 70-80% portion of the group delay characteristic, but the characteristic still exhibits substantial deviations or ears at the band edges. This may be damaging to data signals, even though only a small proportion of the spectrum falls within said ears, since it is these portions of the spectrum that contain the data clock frequency information. The degradations are exacerbated spectacularly when the delay distortions are asymmetric to the signal frequency, i.e. when the filter center frequency drifts under a temperature change.
For these reasons, therefore, it is better to have a flat or nearly-flat group delay characteristic over the entire passband, and a bit more if possible to account for real-world drift of the filter characteristics relative to the signal spectrum. In realizing electric filters, however, the implementation of a sharp frequency selectivity and a flat group delay characteristic generates conflicting design requirements and using conventional elementary prototype filter does not allow to fit both specifications simultaneously.
One solution to fit both of these specifications is to implement a composite filter as described by W. M. Childs, P. A. Carlton, R. Egri, C. E. Mahle and A. E. Williams in "A 14 GHz Regenerative Receiver For Spacecraft Application" (5th International Conference on Digital Satellite Communications, Mar. 20-26, 1981, Genova, Italy, p. 453-459). This composite filter comprises a Bessel bandpass filter and an elliptic bandpass filter operating in cascade via an isolator. The rounded amplitude response and the flat group delay are developed by the Bessel filter and the sharp frequency selectivity feature is developed by the elliptic filter. The authors have disclosed the performance of an exemplary design comprising a 4th degree 72 MHz bandpass Bessel filter with a 6th degree 78 MHz bandpass pseudo-elliptic filter. The major disadvantages of this type of filter lie in its dimensions in view of its cascaded filters interconnected by an isolator, and as a result also in its weight. Both disadvantages become more serious when considered in terms of payload size and weight implications for application in artificial satellites. Furthermore, this type of filter lacks flexibility, since it is limited to particular transmission characteristics. Finally, the filter, in view of its structure equivalent to 10th degree, and its isolator, introduces considerable electrical losses.